1. Field of the Invention
The field of art to which this invention pertains is spent catalyst regeneration. More particularly, the invention relates to a method of initiating essentially complete conversion of CO to CO.sub.2 which method is specifically applicable to spent fluidized catalytic cracking catalyst regeneration.
2. Description of the Prior Art
Regeneration techniques in which a fluidized spent catalyst containing coke is regenerated in a regeneration zone generally occupy a large segment of the chemical arts. In particular the regeneration of a fluidized spent catalyst from a fluidized conversion process such as fluidized catalytic cracking or fluidized dehydrogenation has been quite extensively reviewed by persons interested in those particular processes. The patents which have attempted to solve problems associated with regeneration of spent fluidized catalyst have generally dealt with maximum removal of coke on catalyst while at the same time attempting to prevent or totally eliminate afterburning of carbon monoxide to carbon dioxide within any portion of the regeneration zone.
Specifically it is present refining practice to operate conventional (non-CO-burning) regeneration zones to essentially preclude conversion of CO to CO.sub.2 anywhere within the regeneration zone and especially to preclude afterburning in the dilute catalyst phase where there is little heat sink to absorb the heat of reaction and where heat damage to cyclones or other separation equipment can therefore result. Afterburning in conventional regeneration zones is prevented quite simply by limiting the amount of fresh regeneration gas passing into the regeneration zone. Without sufficient oxygen present to support the reaction of CO to CO.sub.2, afterburning simply cannot occur no matter what the temperatures in the regeneration zone. As well, temperatures in conventional regeneration zones are generally limited to less than about 1250.degree. F. At these temperatures, the rate of reaction of CO oxidation is considerably reduces so that should upsets occur more of an excess of fresh regeneration gas is required for afterburning than would be needed at tempertures higher than about 1250.degree. F. Usual practice, familiar to those skilled in the art of FCC processes, upon starting up a conventional regeneration zone is to manually limit the flow rate of fresh regeneration gas to the regeneration zone to an amount sufficient to produce partially spent regeneration gas but insufficient to sustain afterburning while at the same time limiting regeneration zone temperatures to about 1250.degree. F. This flow rate required is usually equivalent to about 8 to 12 pounds of air per pound of coke. When reasonably steady state control was achieved, it is typical practice to regulate thereafter this flow rate of fresh regeneration gas directly responsive to a small temperature differential between the regeneration gas outlet temperature (or the dilute phase disengaging space temperature) and the dense-bed temperature to maintain automatically this proper flow rate of fresh regeneration gas to essentially preclude afterburning of CO to CO.sub.2 anywhere within the regeneration zone. This practice is exemplified by Pohlenz U.S. Pat. Nos. 3,161,583 and 3,206,393. While such practice produces a small amount of O.sub.2 in the flue gas, generally in the range of 0.1 to 1 vol. % O.sub.2, these prior art processes are operated to preclude essentially complete conversion of CO to CO.sub.2.
Until the advent of zeolite-containing catalysts, there was little economic incentive for essentially complete conversion of CO to CO.sub.2 within the regeneration zone. The heat of combustion that might have been recovered by the process was simply not needed by the process; there was generally no feed preheat for the hydrocarbon reaction zone and the larger coke yield obtained with the amorphous catalysts was generally quite sufficient to provide heat required for the overall process heat balance. The effective utlization of zeolite-containing catalysts with their lower coke-producing tendencies, however, often required an adjustment to the overall heat balance which was normally provided by the addition of a feed preheater. While thermal energy was being added to the front end of the process, the chemical energy of the flue gas exiting from the regeneration zone was often being vented to the atmosphere or being recovered simultaneously in an external CO boiler. Thus a typical flow diagram would then indicate that energy was being added to and then later removed from the process by two external installations, both of which represent a substantial capital investment.
We have now found that it is possible to safely initiate and maintain essentially complete conversion of CO to CO.sub.2 in the regeneration zone. More particularly, we have found that it is possible to initiate and maintain essentially complete conversion of CO to CO.sub.2 within a dense bed of fluidized catalyst located within the bottom portion of a regeneration zone. Our invention is concerned specifically with a method of initiating the essentially complete conversion of CO to CO.sub.2 within a dense phase bed of catalyst maintained in a regeneration zone. As the final step of the method of our invention, coke and CO are oxidized at oxidizing conditions including a temperature from about 1250.degree. F. to about 1400.degree. F. to produce regenerated catalyst having a particular carbon content and spent regeneration gas.
While it is indeed true that prior art references broadly teach the use of temperatures greater than about 1250.degree. F. in regeneration zones (see for example Bunn U.S. Pat. No. 3,751,359; Iscol et al. U.S. Pat. No. 3,261,777; Pfeiffer et al. U.S. Pat. No. 3,563,911, and Lee et al. U.S. Pat. No. 3,769,203) they are concerned with precluding afterburning in regeneration zones and do not teach or suggest the method of our invention for purposefully initiating essentially complete conversion of CO to CO.sub.2 within a regeneration zone. Moreover our method recognizes that essentially complete conversion of CO to CO.sub.2 cannot be initiated by temperatures above about 1250.degree. F. alone; indeed, the method of our invention requires as a distinct step the passing of stoichiometrically sufficient fresh regeneration gas to the dense bed to make possible the essentially complete conversion of CO to CO.sub.2. Without sufficient O.sub.2 present, temperatures higher than about 1250.degree. F. will neither initiate nor sustain afterburning. A temperature of above 1250.degree. F. ensures a sufficiently fast rate of reaction so that conversion of CO to CO.sub.2 will be essentially completed within the dense bed of the regeneration zone.